Hello Gaters I'm trying to simulate a little cold source (say a sphere) placed within a bigger hot source (a huge box). When I create many overlapping sources, the activities are added... so creating a little hot source within a bigger cold source is easy, but creating a little cold source within a bigger hot source is more challenging (this is somewhat related to a question raised on Jan 2006 by Erick Mora but I found no answer on the mailing list...) Is there a simple way to tackle this problem? thanks in advance Jean-Francois